This invention relates to implantable stimulators generally and more particularly to implantable cardioverters and defibrillators.
Early automatic tachycardia detection systems for automatic implantable cardioverter/defibrillators relied upon the presence or absence of electrical and mechanical heart activity (such as intramyocardial pressure, blood pressure, impedance, stroke volume or heart movement) and or the rate of the electrocardiogram to detect hemodynamically compromising ventricular tachycardia or fibrillation. For example, the 1961 publication by Dr. Fred Zacouto, Paris, France, entitled, "Traitement D'Urgence des Differents Types de Syncopes Cardiaques du Syndrome de Morgangni-Adams-Stokes" (National Library of Medicine, Bethesda, Md.) describes an automatic pacemaker and defibrillator responsive to the presence or absence of the patient's blood pressure in conjunction with the rate of the patient's electrocardiogram to diagnose and automatically treat brady and tachyarrhythmias.
Later detection algorithms proposed by Satinsky, "Heart Monitor Automatically Activates Defibrillator", Medical Tribune, 9, No. 91:3, Nov. 11, 1968, and Shuder et al "Experimental Ventricular Defibrillation with an Automatic and Completely Implanted System", Transactions American Society for Artificial Internal Organs, 16:207, 1970, automatically detected and triggered defibrillation when the amplitude of the R-wave of the electrocardiogram fell below a predetermined threshold over a predetermined period of time. The initial system proposed by Mirowski et al in U.S. Pat. No. Re 27,757, which similarly relied upon the decrease in the amplitude of a pulsatile right ventricular pressure signal below a threshold over a predetermined period of time, was abandoned by Mirowski et al in favor of the rate and/or probability density function morphology discrimination as described in Mower et al, "Automatic Implantable Cardioverter-Defibrillator Structural Characteristics", PACE, Vol. 7, November-December 1984, Part II, pp. 1331-1334.
More recently, others have suggested the use of high rate plus acceleration of rate or "onset" (U.S. Pat. No. 4,384,585) with sustained high rate and rate stability (U.S. Pat. No. 4,523,595). As stated in the article "Automatic Tachycardia Recognition", by R. Arzbaecher et al, PACE, May-June 1984, pp. 541-547, anti-tachycardia pacemakers that were undergoing clinical studies prior to the publication of that article detected tachycardia by sensing a high rate in the chamber to be paced. The specific criteria to be met before attempting tachyarrhythmia termination by pacing involved a comparison of the detected heart rate to a preset threshold, such as 150 beats per minute (400 millisecond cycle length) for a preselected number of beats. As stated above, other researchers had suggested the rate of change of rate or suddenness of onset, rate stability and sustained high rate as additional criteria to distinguish among various types of tachyarrhythmias.
Very generally, the systems that depend upon the aforementioned rate criteria are capable of discriminating tachycardia in greater or lesser degree from normal heart rate but can have difficulty discriminating high rate ventricular tachycardias from ventricular fibrillation. In practical applications, a common approach has been to specify discrete rate zones for ventricular fibrillation and ventricular tachycardia, each defined by minimum rates or minimum R--R intervals. However, in some patients, ventricular tachycardia and ventricular fibrillation may have similar rates that make it difficult to distinguish ventricular fibrillation from high rate ventricular tachycardia and supraventricular tachycardias.
Ventricular fibrillation is characterized by chaotic electrical activity which presents highly variable depolarization wavefronts which are propagated in directions which differ from those seen during normal sinus rhythm. Ventricular tachycardias may result from reentry conduction through diseased tissue, which results in depolarization wavefronts, also typically propagated in directions which differ from those seen during normal sinus rhythm. Detection of the occurrence of depolarization wavefronts having directions of propagation which differ from those seen in normal sinus rhythm has been used in various ways in devices intended to detect the presence of ventricular tachycardia or fibrillation.
For example, U.S. Pat. Nos. 3,937,266, 4,088,140 and 4,354,497 describe systems intended to distinguish abnormal ventricular depolarization wavefronts from depolarization wavefronts which originate in the HIS bundle purkinje fiber system. These devices employ a multitude of spaced electrodes coupled to sense amplifiers and attempt to use the relative arrival times of the wavefronts at the various electrodes to detect the occurrence of abnormal conduction.
U.S. Pat. No. 4,754,753 presents a method and apparatus for sensing the probable onset of ventricular fibrillation or pathologic tachyarrhythmias by observing the direction of the depolarization wavefront to predict the onset of harmful ventricular tachyarrhythmias. Detection is accomplished through the use of a multitude of spatially oriented electrodes situated on a pacing lead to provide a vector representation of the direction of propagation of depolarization wavefronts.
Others, such as the inventors of U.S. Pat. No. 4,712,554, have proposed distinguishing between sinus and nonsinus atrial depolarizations by determining the sequence of atrial activation through the use of bipolar or quadrapolar electrodes placed high in the right atrium. U.S. Pat. No 4,577,634 employs quadrapolar atrial and ventricular electrodes for distinguishing retrograde P-wave conduction from normal sinus propagation to avoid pacemaker mediated tachycardia. In a further U.S. Pat. No. 4,790,317, it is proposed to recognize ventricular tachycardia and ventricular fibrillation by comparison of pulse sequences which are obtained when sensing from at least one position on each ventricular epicardial surface. A change in the sequence of activations and in the timing of signals sensed at the two sensor positions is detected and used to indicate either ventricular tachycardia or ventricular fibrillation.
It has also been proposed in the article entitled "Measurement of Difference in Timing and Sequence Between Two Ventricular Electrodes as a Means of Tachycardia Differentiation", by Mercando et al, appearing in PACE, Vol. 9, pp. 1069-1078, November-December, 1986, Part II, that the use of two ventricular sensing electrodes to determine electrical activation sequence in the expectation that the sequence could provide a method for differentiation of normal from abnormal rhythms by implantable antitachycardia devices. Simultaneous recordings from two ventricular sites were obtained during implantation of several devices or programmed electrical stimulation studies. Recordings were made of normal sinus rhythm, ventricular tachycardia, and during premature ventricular contractions. The time intervals between the intrinsic deflections of the two electrograms derived from the ventricular electrodes were measured in a number of the patients and the mean and range values were derived. The authors concluded that the measured mean values of the time intervals over a series of beats could be employed in individual patients to differentiate between normal and abnormal complexes. However, while the authors concluded that it would be feasible to detect differences in sequence timing using two ventricular electrodes in order to distinguish normal sinus beats from ectopic beats, the disclosed range of mean time intervals shows considerable overlap.
Yet another proposal for distinguishing between various types of tachyarrhythmia and ventricular fibrillation is disclosed in U.S. Pat. No. 4,799,493 issued to DuFault. In the device disclosed in this patent, the Widrow-Hoff algorithm is utilized for estimation of a transfer function as a means of discriminating between tachyarrhythmias. The transfer function, once determined generates a replica (estimate) of the signal from a first electrode pair, based on the signal from a second electrode pair. The signal from the first electrode pair can be subtracted from the derived replica (estimate) signal to produce a null signal, in the presence of stable rhythm. Filters specifically tuned to produce null signals in the presence of sinus tachycardia or ventricular tachycardia are disclosed, as well as adaptive filters which automatically converge in the presence of stable rhythm. The automatically adapting filters are disclosed as capable of distinguishing between ventricular fibrillation and tachycardias, in that the LMS algorithms will not allow convergence in the presence of fibrillation. This technique is also described in the article "Dual Lead Fibrillation Detection for Implantable Defibrillators Via LMS Algorithm" by DuFault et al., published in Computers and Cardiology 1986, IEEE Computer Society Press, pp. 163-166.